Re: [PATCH] pidns: remove recursion from free_pid_ns() v5

[Cyrill Gorcunov <gorcunov@openvz.org>]

On Wed, Oct 10, 2012 at 01:54:08PM -0700, Andrew Morton wrote:
> On Thu, 11 Oct 2012 00:42:56 +0400
> Cyrill Gorcunov <gorcunov@openvz.org> wrote:
> 
> > The free_pid_ns function done in recursion fashion:
> > 
> > free_pid_ns(parent)
> >   put_pid_ns(parent)
> >     kref_put(&ns->kref, free_pid_ns);
> >       free_pid_ns
> > 
> > thus if there was a huge nesting of namespaces the userspace
> > may trigger avalanche calling of free_pid_ns leading to
> > kernel stack exhausting and a panic eventually.
> > 
> > This patch turns the recursion into iterative loop.
> > 
> > v5 (from oleg@):
> >  - Drop @ret variable
> >  - Make put_pid_ns non-inline since it grows in size,
> >    in turn make free_pid_ns static
> 
> OK, let's try that.  I'll sit on this until -rc2 to give it a bit of
> time to cook.
> 
> A -stable backport might be needed.  What capabilities does userspace
> need to be able to trigger the kernel stack overflow?

I believe it'll apply on stable even in current form. As Eric mentioned
CAP_SYS_ADMIN is required (so it's not that urgent i think).